1. Field of the Invention
The present invention generally relates to a quadrature modulator. More specifically, the present invention is directed to a quadrature modulator for a digital communication suitable to a phase modulation of a high frequency signal such as in a quasi-microwave signal band.
2. Description of Prior Art
Conventionally, quadrature modulators used in such a high frequency band as a quasi-microwave band have been constructed by way of semiconductor integrated circuits. For instance, "QPSK Modulators for Digital Cellular Communication" of Bipolar Circuits and Technology Meeting 3.2, IEEE, 1992, pages 59 to 62 describes the quadrature modulator as shown in a circuit diagram of FIG. 1.
As represented in FIG. 1, in this prior art quadrature modulator, the QPSK-modulated wave is obtained by phase-shifting the local oscillating signal LO1 by phase shifter 65 to produce a first signal and a second signal having a phase sifted by 90 degrees from that of the first signal. Thereafter, the first signal is supplied to mixer 66 so as to be multiplied with the in-phase signal "I" having the phase opposite to that of this first signal. On the other hand, the second signal is supplied to mixer 67 in order to be multiplied with the quadrature signal "Q" having the phase opposite to that of the second signal. Then, these multiplied output signals are supplied to synthesizing amplifier 68 so as to be synthesized with each other, thereby producing the QPSK modulated wave. After only the required frequency component of this QPSK modulated wave has been filtered out by filter circuit 69 having the bandpass filter characteristic, this filtered frequency component is furnished to mixer 70. In the mixer 70, this filtered frequency component is mixed with another local oscillating signal LO2 to be frequency converted, so that the resultant signal is obtained as the quasi-microwave band signal. Then, further, this quasi-microwave band signal is processed in output amplifier 71 and balance-unbalance converting circuit 72 to be outputted.
Another conventional quadrature modulator, as indicated in a block diagram of FIG. 2, is disclosed in "DIGITAL PHASE-SHIFT QUADRATURE FRONTED FOR LO-INPUTS UP TO 6 GHz" written by P. Weger et. al., Publication of 20th Europe Microwave Conference, 1990, page 426.
As indicated in FIG. 2, 90-degree phase shifter 80 is constructed of mixer 81 and two sets of 1/2 frequency dividers 82 and 83 in the conventional quadrature modulator. In this quadrature modulator, the input signal is multiplied by the local oscillating signal LO by the mixer 81 to obtain the signals having the phases opposite to each other and the frequencies two times higher than that of the input signal. These output signals are divided by 1/2 in the frequency dividers 82 and 83, so that such carrier signals LO.sub.Q and LO.sub.I are outputted, which have the phases different from each other by 90 degrees and also the same frequencies as that of the above-described local oscillating signal LO. Mixers 84 and 85 multiply modulating signals IF.sub.Q and IF.sub.I by the carrier signals LO.sub.Q and LO.sub.I to output the multiplied signals RF as the quadrature modulated waves.
In the conventional quadrature modulator shown in FIG. 2, the 90-degree phase shifter 80 is manufactured by the silicon transistors as the semiconductor integrated circuit. On the other hand, other quadrature modulators manufactured by such compound semiconductors as GaAs as an integrated circuit are conventionally known from, for example, "A 1.9-GHz-Band GaAs Direct-Quadrature Modulator IC with a Phase Sifter" by K. Yamamoto et al., "IEEE JOURNAL OF SOLID-STATE CIRCUITS", VOL. 28, No. 10, October 1993, pages 994 to 1000.
FIG. 3 is a circuit diagram for showing the quadrature modulator manufactured from the compound semiconductor as an IC form, which is disclosed in the above-described publication. This conventional modulator is arranged by 90-degree phase shifter 91, driver 92, and mixers 93 and 94. The driver 92 owns biasing capacitors C1 and C2 at the front stage, and is constructed in such a way that three stages of the differential amplifiers with employment of the field-effect transistor made of GaAs are cascade-connected. The carrier signals having the different phases from each other by 90 degrees are inputted from the 90-degree phase shifter 91 into this driver 92 to be amplified by the driver 92. Then, the amplified carrier signals are supplied to the mixers 93 and 94 in which these amplified carrier signals are multiplied with the modulating signal.
Also, there have been conventionally proposed the methods for correcting the phase errors of the 90-degree phase shifter in, for instance, Japanese Patent Disclosures No. 61-238144 (1986), No. 2-174343 (1990), and No. 4-287542 (1992). As an example, the circuit block diagram of the quadrature modulator described in Japanese Patent Disclosure No. 2-174343 is indicated in FIG. 4. In accordance with the conventional quadrature modulator shown in FIG. 4, the carrier signal entered from input terminal 99 is divided into two divided carrier signals by divider 100. One divided carrier signal is supplied to mixer 103 so as to be multiplied with the first modulation signal supplied from input signal 101, whereas the other divided carrier signal is phase-shifted by 90 degrees in variable phase shifter 105, and the phase-shifted carrier signal is supplied to mixer 104 so as to be multiplied with the second modulation signal derived from input terminal 102. The respective output signals from the mixers 103 and 104 are combined with each other in combiner 108, and then the combined signal is supplied as the QPSK modulated carrier to output terminal 109.
In addition to the above-described general circuit arrangement, phase comparator 106 of this conventional quadrature modulator compares the phases of these divided carrier signals to detect how degree the phase difference between them is shifted from the original 90 degrees. The signal representative of this detected phase difference is supplied via loop filter 107 to the variable phase shifter 105 as the control signal, so that the phase shift amount of the variable phase shifter 105 is variable-controlled in such a manner that the phase difference between the output signals from the mixers 103 and 104 can be equal to 90 degrees.
Further, another conventional quadrature modulator is known from Japanese Electronic Information Communication Institute, Spring Conference C-80 in 1993, in which the 90-degree phase shifter operable in the quasi-microwave band is constructed of the passive circuit operable under no power consumption, as represented in FIG. 5. In this prior art quadrature modulator, inter digital type 90-degree phase shifter 112 is employed on the substrate 111. The carrier signal inputted via terminal 113 is converted by the 90-degree phase shifter 112 into the first carrier signal and the second carrier signal having the different phase from that of the first carrier signal by 90 degrees. The first and second carrier signals are supplied to two-phase modulators 114 and 115, which are manufactured as the integrated circuit, and are modulated by the modulation signals derived from terminals 116 and 117. The signals outputted from the 2-phase modulators 114 and 115 are processed in the combiner formed on substrate 118 and the processed signals are outputted from output terminal 119 as the QPSK ( quadrature phase shift keying ) modulated signal.
In this conventional quadrature modulator, the inter digital type 90-degree phase shifter 112 is formed on the alumina ceramics substrate 111 as the thin film circuit, and both of this alumina ceramics substrate 111 and the substrate 118 are assembled with the two-phase modulators 114 and 115 into a single package, so that this quadrature modulator is operable over such a wide frequency band of 1.5 GHz under less power consumption.
In addition, a further quadrature modulator operable in the quasi-microwave band is known from japanese Patent Disclosure No. 5-347529 opened on Dec. 27, 1993, in which the phase shifter is arranged by the passive element. FIG. 6 is a circuit diagram for showing one example of the phase shifter employed in this conventional quadrature modulator. In this drawing, the 90-degree phase shifter is so arranged that the first phase shifter constructed by cascade-connecting first phase shifting unit 121a and first differential amplifier circuit 122a is connected in parallel to the second phase shifter constructed by cascade-connecting second phase shifting unit 121b and second differential amplifier circuit 122b with respect to the input.
Each of these phase shifting units 121a and 121b is constructed of the series circuit between the two 4-terminal phase shifters. These phase shifters are formed on the semiconductor substrate from the spiral coil functioning as the phase leading element and the MIM (Metal Insulator Metal) capacitor functioning as the phase delaying element. The high frequency input signals RF1 and RF2 having the phases opposite to each other are inputted via the input terminal to these phase shifting units 121a and 121b, and then are phase-shifted by the phase shifting units to produce two sorts of signals S2A, S2B and S4A, S4B which are represented by quadrature vectors. It should be noted in this case that the phase shifting amounts of the phase shifting units 121a and 121b are set in such a manner that combined vector S2 between the signals S2A and S2B is substantially perpendicular to combined vector S4 between the signals S4A and S4B.
The above-described signals S2A, S2B, and S4A, S4B are entered into the gates of the differential-paired transistors in the differential amplifiers 122a and 122b provided at the next stage, and are differentially amplified by these differential amplifiers, whereby the resulting amplified signals are outputted as signals V1A, V1B and V2A, V2B from the in-phase output terminal and the inverse output terminal, respectively. It should be understood that the phase of the output signal V1A is different from that of the output signal V2A by 90 degrees, whereas the phase of the output signal V1B is different from that of the output signal V2B by 90 degrees.
Accordingly to this conventional phase shifter, as indicated in FIG. 7, the signals V1A (V1B) and V2A (V2B) whose phases could be shifted in precision of (90.degree.-2.degree.) to (90.degree.+2.degree.) can be produced with respect to the high frequency input signals RF1, RF2 in the frequency range from 700 MHz to 2 GHz.
As to the above-described conventional quadrature modulator of FIG. 1, after the input signals (local oscillator signals) are modulated by the low frequency signal, the modulated signals are frequency-converted into the quasi-microwave signals in the mixer 70. As a result, there are drawbacks that two sorts of signal sources LO1 and LO2 are required, the complex circuit arrangement is needed, and spurious noise is produced.
As to another conventional quadrature modulator shown in FIG. 2, although only one signal source LO is required, the 90-degree phase shifter manufactured by the bipolar transistor in the semiconductor integrated circuit could not be practically operated with the carrier frequencies higher than 1 GHz even when this 90-degree phase shifter is realized by the circuit arrangement 80 shown in FIG. 2, or other circuit arrangements. Namely, this quadrature modulator cannot be used in the quasi-microwave band. To the contrary, even when this quadrature modulator is operable in such a quasi-microwave band, since active circuit elements are employed, there is another problem that this conventional quadrature modulator would consume high power.
Also, in the conventional quadrature modulator indicated in FIG. 3, since the 90-degree phase shifter 91 utilizes the filter circuit constructed of the resistor and the capacitor, such amplifier 92 whose output level becomes constant is required to correct unbalance in the output levels. Therefore, there is another problem that the power consumption is increased. A further problem exists in that since the field-effect transistor of the compound semiconductor is more expensive than the silicon bipolar transistor, the overall cost of this quadrature modulator would be increased.
Then, another conventional quadrature modulator represented in FIG. 4 employs such a method for improving the phase shifting precision of 90 degrees in such a manner that the phase shifter is constructed of the variable phase shifter 105, and the phase error detected by the phase comparator 106 is fed back to the variable phase shifter 105. However, there is no concrete description about the operation frequency of the variable phase shifter 105 in Japanese Patent Disclosure No. 2-174343, and thus this conventional quadrature modulator cannot be used in the quad-microwave band.
On the other hand, although the power consumption of the conventional quadrature modulator is low, high manufacturing precision is required for the pattern of the thin film circuit used to constitute the inter digital type 90-degree phase shifter 112 on the substrate 111. Accordingly, there are such drawbacks that this quadrature modulator becomes high cost, and such a cumbersome bonding work is required to connect the very narrow patterns to each other by employing such a fine wire as a gold wire. Moreover, there are other drawbacks. That is, since the semiconductor integrated circuit chips of the substrates 111, 118 and the two-phase modulators 114, 115 are connected to the package by the adhesive soldering material, very cumbersome works are needed. In addition, the cost and volume of this quadrature modulator would be increased.
Furthermore, when the phase shifter of the conventional quadrature modulator represent in FIG. 6 is manufactured on the semiconductor integrated circuit, it is difficult to employ such a manner to construct the spiral coil with a desirable constant. Therefore, as a so-called "cut and try" method is necessarily required to manufacture the desirable spiral coil, there is another drawback that a lengthy designing process is needed. Also, since large fluctuation is made in the constants of the spiral coil and the MIM type capacitor due to the integrated circuit itself, the resulting 90-degree phase difference is greatly fluctuated. Moreover, when such a spiral coil having the constant of several hundreds nH is constructed, a large area is required, which may cause problems that the overall volume of this quadrature modulator is increased and the high cost thereof is needed.
Additionally, as apparent from the characteristic shown in FIG. 7, the phase shifter of the conventional quadrature modulator owns errors with +2.degree. to -2.degree. in the frequency range from 700 MHz to 2 GHz. Taking account of the fluctuation, the entire-errors would be further increased. Further, such a band width of 1.3 GHz where a desired phase shift could be achieved would be an insufficient band width in view of various usage in the quasi-microwave band.